With more than 10 million people affected and 24 million people at risk, osteoporosis has a significant impact on the U.S. healthcare system. Osteoporosis is associated with decreased bone mass and deterioration of the trabecular architecture of the bone, which collectively impact the mechanical properties of the bone. Traditionally, measuring bone mineral density (BMD) has been the predominant diagnostic and screening tool for osteoporosis and other degenerative bone diseases. Currently, dual energy x-ray absorptiometry (DXA) is the most common method of assessing bone density. However, this method is costly. Further, the level of success using the BMD method relies on the validity of a theory that indirectly relates BMD to energy absorption. In other words, no actual energy absorption process is involved with the BMD method. Properties of a bone that manifestly relates to mechanical energy absorption such as vibration damping may be measured instead. In such a case, a theory establishes differential equations of motion that explicitly place bone vibration damping in the role of absorbing energy, and the damping may be directly measured in situ.
The human body is subjected to constant loading and impact during normal daily activities. Among the natural shock absorbers in the human body, trabecular bone has the highest capacity to attenuate incoming shock waves associated with, for example, walking and running. Since osteoporosis is associated with decreased bone mass and deterioration of trabecular architecture of the bone, the disease detrimentally changes the bone's natural shock absorbing capacity.
A conventional procedure for assessing the dynamic bone quality of the trabecular bone involves striking the heel against a force pad and measuring the damping values associated with resonant vibrations in the frequency range of interest. The heel strike induces vibration over the frequency range from about 10 Hz to about 100 Hz (sometimes, but not consistently, extending to 200 Hz). The shock absorption may be quantified by determining a damping ratio of the tibia, which is a measure of the structural integrity of the bone. The damping ratio may be compared to a reference value representing healthy individuals to assess the dynamic bone quality. Generally, a damping ratio that is lower than the reference value indicates a presence or risk of bone disease.
Accurately determining a damping ratio is made difficult due to the vibration of tissue surrounding the tibia. The damping ratio generally used to indicate a healthy tibia is around 35 percent. However, it has been shown that the damping ratio of a healthy tibia without interference from any surrounding tissue is about 10 percent. A metric that includes the effects of surrounding tissue may be representative of the total osteoporosis condition. However, the wide variation within a patient population in the characteristics of tissue surrounding the tibia introduces significant uncertainty in whether the damping ratios produced by conventional procedures adequately represent the dynamic bone quality. Thus, there is an increasing need to provide improved non-invasive, economical tools for accurately assessing and monitoring dynamic bone quality. More particularly, there is a need for improved methods of measuring the damping ratio while the tibia is vibrating in a mode that is decoupled from surrounding tissue (i.e., removed from the influence of surrounding tissue).